Many natural crystalline materials—quartz is an example—exhibit piezoelectric properties—that is, when a piece is squeezed it develops a significant electric potential across the relevant opposite faces. A typical example of this is the spark generator in some gas or cigarette lighters. The opposite effect is also well known; pieces of such materials if subjected to an electric field in an appropriate direction very slightly change dimensions (expanding or contracting, usually observed as a lengthening or a shortening, depending on the field direction). A typical example of this is an ultrasound generator as used in medical body scanning devices. For shape-changing piezoelectric devices which, like these, utilise a single piece, or layer, of piezoelectric material the range of shape changing is usually extremely small—possibly no more than 0.01 mm (10 micrometre).
Much interest is presently being shown in the use of piezoelectric materials to construct actuators for a wide variety of mechanical devices, ranging from loudspeaker drivers to camera lens focusing systems, from electric toothbrushes to computer disk drive head positioners, and from aircraft-wing de-icers to gas valve controllers. Many, even most, of these actuators take the form of a device known as a “bender”—either a composite of a single thin flat elongate layer (like a plank) of piezoelectric material and a similar but inactive layer bonded together face to face to form a beam (a unimorph) and provided with activation electrodes, or a composite of two such piezoelectric material layers similarly bonded (a bimorph) and with electrodes both on each external face and also between the two bonded faces.
During manufacture, such devices are poled—that is, the layers are subjected to a very high electric field. Subsequently, in use the layers are activated by using the electrodes to apply an electric field across the piezoelectric layer, causing the piezoelectric layer to expand/lengthen (or contract/shorten, depending on the field's polarity relative to the poling direction). When such a bimorph device is activated (by using the electrodes to apply an electric field across each layer, the two fields being of the opposite polarity relative to the poling direction), one layer expands/lengthens while the other contracts/shortens. In each case, because of the spatial separation of the device's two layers either side of the joint face (the median plane) the composite is caused to bend (in the case of an elongate beam device one end moves up or down, or back or forth, relative to the other)—and this bending can provide movement and force so as to actuate some suitably-connected machinery. Benders of this two-layer shape-changing sort are capable of providing quite considerable movement, though even the best tend to be restricted to a millimetre or so.
Benders, especially bimorph benders, work well in many situations, but because of the need to have, and connect to, their central electrode they are not as easy to make and use as might be desired.
Examples of known benders and actuators are described in U.S. Pat. No. 3,816,774, in which various, mostly bimorph-type, structures are described. Single-layer serpentine structures are described in U.S. Pat. No. 4,028,566; U.S. Pat. No. 4,284,921 and U.S. Pat. No. 5,633,554 and WO-99/05778. Polymeric piezoelectric transducers of various shapes are described in U.S. Pat. No. 4,056,742 and U.S. Pat. No. 4,284,291.
It would be highly advantageous to be able to do away with the central electrode of bimorph benders, and thus in effect have a relatively simpler one-layer device. Unfortunately, it has hitherto not been possible to achieve with a single layer device the relatively large amounts of shape-change movement attainable using the bending ability of a bimorph or even a unimorph.